Haskell <-> C/C++ comunication (sockets/pipes?)

Can you tell me what is the problem here?
Haskell app output is redirected to C++ input app (and vice-versa) using
pipes. (used Erays shell script sugestion)
When I try
Hsk: 1. get, 2.send
C++: 1. send, 2 get
It works fine, but if I try it the other way around it locks.
C++ app won't get any input.
Seems to me like Hsk is not 'flushing' the output.
If this is the case, I can't really explain why it happens... if the output
was not redirected I am pretty sure that, before trying to get input the
output would be flushed
(imagine something like putStrLn "please type an int:")
So what is the issue here?
Here is the code,
And once again, thanks in advance.
J.A.
-------------------------------
Haskell app: send -> get
-------------------------------
main :: IO()
main = do
--send hsnum
putStrLn "6"
-- get cnum and save
num <- readLn :: IO(Int)
writeFile "./reshs.txt" (show num)
----------------------------
C++ app: get -> send
----------------------------
#include <cstdio>
#include <iostream>
#include <fstream>
nt main(){
// *get* hsnum and save
int num;
ofstream out("rescc.txt");
cin >> num;
out << num;
// *send* cnum
cout << 10 << '\n' << flush; //it works without the flush
return 0;
}